There are several techniques known for reducing the charge interaction between adjacent droplets in an ink jet printing system. U.S. Pat. No. 3,562,757 of Bischoff, describes an ink jet system wherein charge interaction between adjacent droplets and aerodynamic drag is compensated for. The compensation comprises utilizing the "guard drop" principle in which every other droplet is charged, such that every other droplet is guttered thereby effecting an increase in distance between the droplets which are used for printing, thereby reducing the charge interactions between printing droplets as well as the wake between the droplets used for printing. In Bischoff there is no aspiration used, and the efficiency of the system is decreased due to the guttering of an excessive number of droplets.
The concept of utilizing a gas stream, such as air, to compensate for aerodynamic drag in an analog deflected ink jet system is set forth in U.S. Pat. No. 3,596,275 of Sweet. Sweet introduces a colinear stream of air, used to reduce the effects of the wake of a given droplet relative to a following droplet, with the objective being to remove the drag on each droplet. However, in Sweet, the gas stream becomes turbulent before it matches the drop velocity. In Sweet, the ink jet nozzle is mounted on an airfoil-like structure which is placed near the center of a wind tunnel where the air stream is accelerated to near maximum velocity. Since even a good airfoil has a small but unstable wake which is swept along with the ink droplets the droplets trajectory of Sweet is affected by the wake and accordingly optimum minimization of aerodynamic distortion is not achieved.
U.S. Pat. No. 3,972,051 of Lundquist et al discloses an ink jet printing system which includes a laminar airflow passageway through which ink droplets are directed before striking a moving print medium. The airflow is created by suction at the downstream end of the passageway, with the airflow not being filtered before it enters the passageway. Accordingly, aerodynamic disturbance of the airflow might be created by the air passing over the charge electrode and deflection electrodes. The geometry of the entrance and exit apertures of the passageway is rectangular, with the passageway having a non-uniform cross-sectional area, with the laminar flow of the air having a non-constant velocity and being reduced in velocity as the airflow approaches the print medium. Here too, the air velocity is everywhere only a fraction of the droplet velocity to avoid turbulence.
U.S. Pat. No. 4,097,872 to Giordano et al., which is assigned to the assignee of the present invention, discloses an aspirator for an ink jet printing system in which the aspirator includes a passageway, such as a tunnel, having a constant cross-sectional area, and in which the velocity of the airflow therethrough is substantially constant and equal to the ink droplet velocity such that the aerodynamic drag on the droplets is substantially eliminated.
Although the art above discloses air aspiration systems, none of this art sets forth any air servo systems for an ink jet aspirator for maintaining a constant air velocity input to the aspirator. Systems for the control of airflow, are however, generally known. One such system, is set forth in U.S. Pat. No. 3,425,278 to Buzza, which discloses a flow meter which controls the flow of air through a first pipe to match the flow of air through a second pipe, with the control flow of air through the first pipe being indicative of the rate of flow through the second pipe. U.S. Pat. No. 1,920,752 to Kissing et al discloses a fluid pressure regulator for prime movers and the like, whereby actuations of the regulator due to changes in viscosity or like physical characteristics of the actuating fluid are compensated for.
None of the above cited art discloses a fluid velocity measuring system as described herein, or suggests the use of an air servo system for controlling the velocity of airflow in an enclosure such as an ink jet aspirator to a constant velocity by sensing either the velocity or pressure therein and comparing it with a reference velocity or pressure, respectively. According to the present invention, such an aspirator air servo system is set forth, wherein a precisely controlled air velocity and pressure are provided by a reference air source whose frequency is derived from a crystal oscillator. The total air pressure from the aspirator wind tunnel is compared with the total air pressure from the reference air source using a matched thermistor pair technique to convert the pressure difference into an electrical error signal which is used to control the main air source for the aspirator wind tunnel such that the error signal is maintained at zero thereby maintaining the air velocity in the aspirator constant.